1. Field of the Invention
This invention relates to a premixed compression ignition type internal combustion engine, adapted to reduce a concentration of NOx discharged from a diesel engine (compression ignition type internal combustion engine) and increase an output level thereof.
2. Background Art
FIG. 6 is a time chart showing a comparison between the fuel injection time of a regular diesel engine and that of a premixed compression ignition type internal combustion engine, and FIG. 7 is a graph showing a comparison between a concentration of NOx discharged from the regular diesel engine and that of the premixed compression ignition type internal combustion engine.
As shown in FIG. 6(a), in the conventional diesel engine, a needle valve in a top portion of a combustion chamber is opened when a piston is in a position in the vicinity of a compression top dead center TDC, and fuel is injected (normal injection) into the combustion chamber. In such a conventional diesel engine, NOx of a comparatively high concentration is discharged for the following reasons.
The concentration of discharged NOx becomes highest under the condition that the fuel and air react with each other neither excessively nor insufficiently, i.e., when an excess air ratio .lambda. reaches a level in the vicinity of 1. Where the fuel is injected into a combustion chamber when the piston is positioned in the vicinity of the compression top dead center TDC in the conventional diesel engine, the fuel is burnt (diffusion combustion) as it is mixed with the ambient air and diffused into the interior of the combustion chamber. Accordingly, a region in which the excess air ratio .lambda. necessarily becomes 1 exists in this case in a process in which the fuel is mixed with the air, and NOx of a concentration higher than that in this region is discharged. Moreover, when an engine output level is increased by increasing, as shown in FIG. 7, the quantity of the fuel supplied to the combustion chamber, the concentration of discharged NOx increases in proportion to the increase in the quantity of the fuel.
Therefore, at present, a premixed compression ignition type internal combustion engine capable of greatly reducing the concentration of discharged NOx, as compared with such a conventional diesel engine, has been studied and developed. As illustrated in FIG. 6(b), the premixed compression ignition type internal combustion engine is an engine in which a fuel is injected early (when a piston is in the vicinity of a compression bottom dead center BDC) into a combustion chamber so as to finish the supplying of the fuel into the combustion chamber at least before the piston reaches the compression top dead center TDC. The fuel injected early into the combustion chamber is mixed sufficiently with the air in the combustion chamber by the time the piston moving from the fuel injection time has arrived at the compression top dead center TDC, the fuel being thereby put in a lean mixed state (lean state)
Namely, in the premixed compression ignition type internal combustion engine, the fuel and air are mixed with each other sufficiently, and they are put in a lean mixed state before the fuel is self-ignited by a compression effect of a piston when the piston reaches a position in the vicinity of a compression top dead center TDC. This gaseous mixture in a lean mixed state is compressed by the piston, whereby the fuel is self-ignited (multi-point simultaneous compression self-ignition combustion) simultaneously in multiple points in the combustion chamber when the piston is in the vicinity of the compression top dead center TDC. There is another premixed compression ignition type internal combustion engine in which a fuel is injected into a suction pipe and turned into a gaseous mixture, which is taken into a combustion chamber in a suction stroke.
In any case, in a premixed compression ignition type internal combustion engine, the fuel and air are mixed with each other in advance so as to put them in a lean mixed state in which an excess air ratio .lambda. is sufficiently larger than 1, and this gaseous mixture is thereafter burnt (lean combustion), so that the gaseous mixture is not burnt when the excess air ratio .lambda. is 1. Accordingly, in a premixed compression ignition type internal combustion engine, the concentration of discharged NOx decreases greatly as shown in FIG. 7 as compared with that in the conventional diesel engine, and extra-low NOx combustion is attained.
Especially, when light oil is used as a fuel in a premixed combustion ignition type internal combustion engine, a compression ratio is set lower than that in a regular diesel engine, whereby the time, at which a lean gaseous mixture is self-ignited, is regulated so that the self-ignition time is close to the time at which a piston reaches a compression top dead center TDC. The reason resides in that, when both the compression ratio and temperature are high, there is a fear of the occurrence of self-ignition of the lean gaseous mixture before the piston reaches a position in the vicinity of a compression top dead center TDC.
However, in the premixed compression ignition type internal combustion engine of the related art, a high load operation (high output operation) cannot be carried out since a decrease in the concentration of discharged NOx is attained by lean combustion.
Concretely speaking, in the case of a premixed compression ignition internal combustion engine, a ratio of the quantity of the air supplied to the interior of a combustion chamber to that of a fuel supplied thereto is satisfactory as shown in FIG. 7 in a low and intermediate load operation with respect to a purpose of reducing the concentration of discharged NOx by premixed compression ignition combustion. However, when an engine output is increased by increasing the quantity of fuel supplied to the combustion chamber, knocking occurs in a region (high load region) of an output level not lower than a certain level P, and the operation of the engine becomes difficult. The reason resides in that the air, the quantity of which corresponds to an increase of the quantity of the fuel, is not obtained when the output level is not lower than the level P, whereby an excess air ratio .lambda. comes close to 1. When knocking occurs, the concentration of discharged NOx increases up to a level not lower than a level usual in a regular diesel engine, and the vibration of the engine increases. This would cause engine trouble to occur.
In the premixed compression ignition type internal combustion engine, a control operation is carried out so that a lean mixed state can be maintained even when an output (load) increases, by setting the fuel injection time to an earlier time in accordance with an increase in the quantity of the fuel. In a suction stroke of the engine, the quantity of the air sucked due to a negative pressure in the combustion chamber in a suction stroke of the engine is substantially constant, so that, after all, the quantity of the air corresponding to an increase in the quantity of the fuel is not obtained in a region in which the engine output is not lower than a certain level. Consequently, the excess air ratio .lambda. necessarily comes close to 1.
Even when the quantity of a fuel is increased in a conventional diesel engine until an excess air ratio .lambda. reaches, for example, around 1.4, the engine can be operated without causing knocking to occur. However, in a premixed compression ignition type internal combustion engine, a lean combustion operation is carried out so as to reduce concentration of discharged Nox. Therefore, for example, in a basic test using a single-cylinder and not using an EGR (Exhaust Gas Recirculation) system, an operation limit exists at an excess air ratio .lambda. of around 2.0-2.3. Therefore, the premixed compression ignition type internal combustion engine is inferior to a conventional diesel engine of the same displacement.
In view of the above-described related techniques, the present invention aims at providing a premixed compression ignition type internal combustion engine capable of retaining a lean mixed state by securing the quantity of the air which corresponds (not causing knocking to occur) to that of the fuel supplied to the interior of the combustion chamber, even at a high load time; and expanding an output range with extra-low NOx combustion ratio maintained.
Although Japanese Patent No. 2812236 discloses techniques concerning a premixed compression ignition type internal combustion engine, a supercharger like that used in the present invention is not provided. Therefore, an operational region (for example, a high load region) in which the quantity of the air is insufficient with respect to that of the fuel) exists, and, in this operational region, knocking occurs in some cases.